I remember an occasion when I was young when my dad and brother and I took a flight on a small regional plane, probably on one of our annual ski vacations. The little plane was a turboprop, and it was the first time I’d been on a plane with actual propellers—every other commercial flight I’d had at that point in my young life had been on jets. Already a nervous traveler, I’d told my dad that I hated the idea of our flight being dependent on the two visible fast-spinning props. A jet engine, with its cowling and apparent motionless magic, seemed a lot more sensible way to provide thrust.

"Fast spinning propellers?" my dad said. "When you look at a jet engine, what do you see in the front? A big fast-spinning turbine!" I realized he was right—jet engines weren’t immutable chunks of high-technology. They had a giant fan at the front, pushing air through just like a propeller! This probably triggered some kind of panic attack and I probably started crying or something, because I was pretty whiny about flying.

Still, it was eye-opening. When I visited GE’s Global Research Center in Munich last month, I heard quite a bit about how GE’s aviation division makes jet engine turbine blades out of carbon fiber—which seemed like the perfect branching off point for an explainer about the different types of jet engines. And it surprised me just exactly how insightful my dad’s long-ago comment turned out to be. As it turns out, a high-bypass turbofan like you’d find on most modern airliners really is like a giant propeller.

Enlarge/ GE's Dr. Carlos Härtel shows off a GE-produced carbon fiber turbofan blade at left, comparing it to a turboprop blade at right.

Turbojet, turbofan—turbo turbo turbo!

Attempting to actually enumerate and walk through all the different kinds of jet engines would take probably ten thousand words, minimum. That’s not the goal here. However, there are two main jet engine variants we are going to describe that will cover the gamut of mainstream and commercial and military aviation: turbojets and turbofans.

But before we get into the differences between a turbojet and a turbofan, it’s important to talk about the ways in which they’re alike. A turbojet and a turbofan both operate on similar principles: air enters the front of the engine through an intake, and some or all of the air is compressed, mixed with fuel, burned, and then expelled to impart propulsive force. While a big external propeller thrashes its way through the air, a jet engine does much the same—it just does it within an enclosed nacelle or fairing.

The "turbo" component of the engine refers to the gas turbine at the heart of each type of engine. This section of the engine actually drives the other components, and it also extracts energy from the engine to power the aircraft’s systems (electrical, hydraulic, and others as well).

At first blush, a jet engine appears to represent a chicken-and-egg problem, especially in a high-bypass engine. Air enters through the intake and is pushed along by the front fan, compressed and heated and ignited by the middle stage, then expelled out the back. The combustion in the middle stage, in turn, provides power to the front fan to pull in more air. The reason it’s not a circular chicken-and-egg problem is the energy added into the mix by the ignition of fuel.

Starting a jet engine necessarily means getting the entire system moving: pulling in enough air to drive the compressor, driving the compressor enough to spin the fan, spinning the fan enough to pull in the necessary air to drive the compressor ad infinitum. This problem is worked around in planes most often by using pressure from a smaller jet engine called the APU (for "auxiliary power unit"); an aircraft’s APU generates power on the ground when the main engines aren’t turning and also provides energy in the form of electricity, hydraulic pressure, or pneumatic pressure to get the main engines turning enough to ignite and run on their own.

Turbojets

The first kind of jet engine that came into regular service was the turbojet. In a turbojet, there isn’t any concept of "bypass" air; all the air entering into the engine is fed into the compressor. When jet engines were first flown in the 1930s, this design was the simplest to engineer and field. It certainly made an impression on the Allies when the turbojet-powered Messerschmitt Me 262 started flying combat sorties in 1944.

Enlarge/ Schematic of a turbojet engine. Air enters, is compressed and juiced with fuel, gets ignited, and is expelled propulsively as exhaust.

But the turbojet design has a number of limitations. First, although simpler to design, the actual engines that were built by various manufacturers around the world proved to be expensive to build and even more expensive and complex to maintain. The fact that the entirety of the engine’s propulsive energies come from the ignition of a fuel-air mixture also means that turbojets consume prodigious quantities of gas. They also lack efficiency at lower speeds.

Turbojets began to be phased out in common usage for all but high-speed applications, and by the 1970s even small military supersonic fighters had shifted over to a much more efficient design: the turbofan.

Turbofans

A turbofan operates somewhat like a turbojet, but there's one major difference: not all of the air gets pulled from the inlet into the compressor. Instead, some amount of air—called the "bypass" air—is pushed directly out of the exhaust section by the front fan. In this way, a turbofan engine’s propulsive effect comes from both the combustion stage and also from the bypass air.

There are two major variations of turbofans—low-bypass and high-bypass. The names are descriptive: a high-bypass turbofan channels the majority of the air from its inlet into the bypass duct and has a core turbine stage that’s relatively small compared to the overall size of the engine, while a low-bypass turbofan has a larger core stage and ducts considerably more air into it rather than letting it bypass the core. The ratio of air shunted into the bypass duct versus the turbine core is called the "bypass ratio," and an engine is classified as "high-bypass" or "low-bypass" depending on which side of the ratio it falls.

Enlarge/ A high-bypass turbofan. Far more air is channeled around the compressor through the bypass stage than is actually ignited. This makes for quieter, more fuel-efficient operation.

Each design has its advantages. Low-bypass turbofans are most often seen in small jets, especially military fighters. They require less physical space and still give gains in efficiency over a pure turbojet design. On the other hand, high-bypass turbofans are found on basically every modern jet-powered commercial airliner in service today—every engine slung under the wing of every big commercial jet is a high-bypass turbofan from one major manufacturer or another.

Enlarge/ A low-bypass turbofan, where more air is directed into the compressor than into the bypass portion. Low-bypass turbofans can be smaller than high-bypass; most modern fighter jets are powered by low-bypass turbofans.

Aside from drastically increased fuel efficiency over a turbojet, high-bypass turbofans can be significantly quieter than turbojets or low-bypass turbofans. This is very much a consideration in commercial aviation, where decreasing the total amount of noise generated from an airplane can lead to larger and denser airports around cities and towns without adjustments to noise laws or regulations.

And so much more

Although turbojets and turbofans cover the majority of jet engines used today, there are many more fascinating types of air-breathing engines pushing jets through the sky. Of particular interest are ramjets and scramjets, which generate jet propulsion without the need for big clunky spinning blades or turbines. Of particular historical interest is the Pratt & Whitney J58 turbojet, the hybrid engine that powered the Blackbird series of jets. It ran as a turbojet up through high Mach speeds and then converted itself into a partial ramjet, relying on a complex system of moving ducts and inlets to regulate the flow of air.

But exotic variants aside, the turbofan is the workhorse of modern jet-powered aviation. So next time you’re sitting on the runway awaiting takeoff, take a moment to glance at that engine pod underneath the wing. The big fan in the front is providing a lot more juice than you might have thought.

Share this story

Lee Hutchinson
Lee is the Senior Technology Editor at Ars and oversees gadget, automotive, IT, and gaming/culture content. He also knows stuff about enterprise storage, security, and human space flight. Lee is based in Houston, TX. Emaillee.hutchinson@arstechnica.com